Field of the Invention
The present invention concerns a process for producing porous preform webs from cellulosic fibres and thermoplastic materials by foam forming, and a process for moulding these composite foams into the desired rigid structures.
Description of Related Art
Rigid moulded structures are commonly manufactured from composites, due to the strength, light weight and low costs of these. Polymer composites include fibre-reinforced polymeric materials. The polymer component can be a thermoset or a thermoplastic component. The reinforcement, in turn, typically consists of glass fibres, carbon fibres, cellulose (such as wood fibre, flax or hemp) or high strength polymers.
The existing solutions are produced, for example by using fibrillated polymers, typically polylactic acid (PLA), or a synthetic thermoplastic polymer, such as polypropylene (PP), and a woven or non-woven fibre web, or a glass-fibre or agrofibre web.
FR 2932511 A utilizes one of these typical polymer types, polylactic acid (PLA), in forming a fibrous structure that traps natural fibres into a 3-dimensional network. The obtained material is shaped into thermal insulation panels by blending the components, e.g. by extrusion, followed by casting them onto a suitable support and carrying out a heat treatment to cause adhesion to seal the structure. Using this procedure, only sheet-like structures can be obtained.
A more versatile and thus preferred moulding technique for such reinforced thermoplastics is injection moulding. Utilization of composites with high cellulose content in structural solutions is, however, challenging by both injection moulding and extrusion due to the fibre damage and the resulting degenerated product performance commonly caused by this technique in these materials.
If lignin would be used as a thermoplastic component in composites, said problem would be emphasized due to the low molecular weight of the lignin, and the high viscosity of the resulting composite. Thus, lignin composites with high cellulose contents are not common.
U.S. Pat. No. 6,756,114 B instead relates to a composite formed from synthetic cellulose fibres and a synthetic thermoplastic polymer, which is pelletized before moulding. According to this publication, the moulding can be carried out either by injection moulding or compression moulding. The pelletizing procedure, however, includes a step that resembles extrusion or coating, and constitutes quite an extensive pre-treatment, yet without providing a homogeneous mixture of fibre and polymer.
US 2006/099393 relates to a composite formed of natural cellulose fibres and a thermoplastic resin. The natural fibres and the thermoplastic are dispersed in an aqueous medium.
Södra Cell is, in turn, marketing a composite material called DuraPulp (see http://www.sodra.com/en/pulp/pulp_products/Composite-material/DuraPulp/), which includes wood fibres and polylactic acid (PLA). The material can be wet-formed and moulded, but this requires the use of high water-contents, which in turn results in high energy consumption during the drying step. Further, these wet-forming techniques do not allow the use of light thermoplastic polymers (lighter than water) in powder or granulate form. Additionally, the wet-formed structure is relatively dense, resulting in limited compression mouldability.
Thus, there exists a need for processes that can provide high-strength cellulosic composite structures without requiring extensive pre-treatments or high water contents, while being easy to process with conventional heat-moulding techniques, such as compression moulding.